I want to design a passive 4-pole low-pass RLC filter for my subwoofer amplifier. So far I found this page: http://sim.okawa-denshi.jp/en/RLClowkeisan.htm But it seems that changing the value of R does nothing to the cutoff frequency. I think this is strange and would like to clarify this.
Also, I'm confused about the impedances to consider when designing low-pass filters. Do I include the source impedance only? PC Sound Card output impedance is 50 Ohms I guess? Or do I also include the load impedance, which in my case is the amplifier input, and is 30k? I'm confused.
I'd like a good (true) online calculator or a design tool for 4-pole RLC filters, or at least an explanation.
I forgot to mention this, but I'm planning to cascade the filters through an amplifier. This is different than using an active filter on opamps, but I work with what I have. Basically I have a stereo amp and one of its channels I'm going to use for amplifying between the two 2-pole filters. Still I dont get how the linked page suggests the cutoff frequency is same for different impedances. Somehow I don't think I can use the same micro filter after the final amp output where the signal is powerful and load impedance is low. Intuitively, I'd need much bigger inductors and capacitors. The page doesn't reflect that